Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide methods and apparatuses for video encoding/decoding. In some examples, an apparatus for video decoding includes processing circuitry. The processing circuitry receives coded information of a current coding tree unit (CTU) from a coded video bitstream. Then, the processing circuitry determines a context model for a split flag associated with a current block within the current CTU at least partially based on split information of a corresponding block in a reference CTU for the current CTU. The split flag associated with the current block is indicative of split information of the current block. Then, the processing circuitry determines the split flag based on the context model, and decodes the current block based on the split flag that is determined based on the context model.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/885,728, “METHODS FOR IMPROVED CONTEXTMODELING IN BLOCK STRUCTURE SIGNALING FOR AVS” filed on Aug. 12, 2019,which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has specific bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth and/or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless compression and lossy compression, as well as a combinationthereof can be employed. Lossless compression refers to techniques wherean exact copy of the original signal can be reconstructed from thecompressed original signal. When using lossy compression, thereconstructed signal may not be identical to the original signal, butthe distortion between original and reconstructed signals is smallenough to make the reconstructed signal useful for the intendedapplication. In the case of video, lossy compression is widely employed.The amount of distortion tolerated depends on the application; forexample, users of certain consumer streaming applications may toleratehigher distortion than users of television distribution applications.The compression ratio achievable can reflect that: higherallowable/tolerable distortion can yield higher compression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is using reference data only from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102) that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples 541, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (180) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 2, a current block (201) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (202 through 206, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes processing circuitry. The processing circuitry receives codedinformation of a current coding tree unit (CTU) from a coded videobitstream. Then, the processing circuitry determines a context model fora split flag associated with a current block within the current CTU atleast partially based on split information of a corresponding block in areference CTU for the current CTU. The split flag associated with thecurrent block is indicative of split information of the current block.Then, the processing circuitry determines the split flag based on thecontext model, and decodes the current block based on the split flagthat is determined based on the context model.

In some embodiments, the split flag can be any one of a quad tree (QT)split flag indicating whether a QT split is used; a binary or extendedquad tree (BET) split flag indicating whether a BET split is used; a BETsplit type flag indicating a split type selected from a binary tree (BT)split and an extended quad tree (EQT) split; and a BET split directionflag indicting a direction selected from a vertical direction and ahorizontal direction.

In an embodiment, the processing circuitry determines the reference CTUto be a spatial neighbor of the current CTU. In another embodiment, theprocessing circuitry determines the reference CTU with a split structurestored in a history buffer. In another embodiment, the processingcircuitry determines the reference CTU in a different picture from thecurrent CTU. In another embodiment, the processing circuitry determinesthe reference CTU based on information in a high level header.

In some embodiments, a relative position of a top left corner of thecurrent block with reference to a top left corner of the current CTU isthe same as a relative position of a top left corner of thecorresponding block with reference to a top left corner of the referenceCTU.

In some embodiments, the processing circuitry selects the context modelfrom a first set of context models in response to the split informationof the corresponding block indicating further splitting of thecorresponding block and selects the context model from a second set ofcontext models in response to the split information of the correspondingblock indicating no further splitting of the corresponding block.

In some embodiments, the processing circuitry selects the context modelwith a largest possibility for further splitting the current block inresponse to the split information of the corresponding block indicatingfurther splitting of the corresponding block.

In some embodiments, the processing circuitry disables a usage of thesplit information of the corresponding block in the determination of thecontext model in response to a partition depth being deeper than athreshold.

In some examples, the processing circuitry determines the splitinformation of the corresponding block based on a two levels quad treesplit structure with a first bit indicting QT splitting information ofthe reference CTU and four other bits respectively indicating splittinginformation of four sub blocks of the reference CTU.

In an embodiment, the processing circuitry determines the context modelfor the split flag based on a slice type of the current CTU.

In some examples, the processing circuitry determines the context modelfor the split flag based on the reference CTU in response to a splitdepth of the current CTU is below a threshold.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the methodsfor video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes.

FIG. 1B is an illustration of exemplary intra prediction directions.

FIG. 2 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 6 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 7 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 8 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 9 shows examples of BT splits, TT splits and EQT splits accordingto embodiments of the disclosure.

FIG. 10 shows a flow chat outlining a partition process used in AVSaccording to some embodiments.

FIG. 11 shows a syntax table for block structure signaling according tosome embodiments.

FIG. 12 shows an example of using temporal neighboring CTU(s) to predictthe current block split structure for a current CTU according to anembodiment of the disclosure.

FIG. 13 shows an example of a block split structure modificationaccording to an embodiment of the disclosure.

FIG. 14 shows an example for representing the two-level QT splitstructure according to an embodiment of the disclosure.

FIG. 15 shows a flow chart outlining a process according to anembodiment of the disclosure.

FIG. 16 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the FIG. 3 example, the first pair of terminal devices (310) and(320) performs unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (330) and (340)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (330) and (340) via the network (350). Eachterminal device of the terminal devices (330) and (340) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (330) and (340), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 3 example, the terminal devices (310), (320), (330) and(340) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (350) represents any number ofnetworks that convey coded video data among the terminal devices (310),(320), (330) and (340), including for example wireline (wired) and/orwireless communication networks. The communication network (350) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(350) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (413), that caninclude a video source (401), for example a digital camera, creating forexample a stream of video pictures (402) that are uncompressed. In anexample, the stream of video pictures (402) includes samples that aretaken by the digital camera. The stream of video pictures (402),depicted as a bold line to emphasize a high data volume when compared toencoded video data (404) (or coded video bitstreams), can be processedby an electronic device (420) that includes a video encoder (403)coupled to the video source (401). The video encoder (403) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (404) (or encoded video bitstream (404)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (402), can be stored on a streamingserver (405) for future use. One or more streaming client subsystems,such as client subsystems (406) and (408) in FIG. 4 can access thestreaming server (405) to retrieve copies (407) and (409) of the encodedvideo data (404). A client subsystem (406) can include a video decoder(410), for example, in an electronic device (430). The video decoder(410) decodes the incoming copy (407) of the encoded video data andcreates an outgoing stream of video pictures (411) that can be renderedon a display (412) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (404),(407), and (409) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anembodiment of the present disclosure. The video decoder (510) can beincluded in an electronic device (530). The electronic device (530) caninclude a receiver (531) (e.g., receiving circuitry). The video decoder(510) can be used in the place of the video decoder (410) in the FIG. 4example.

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (501), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (531) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (531) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (515) may be coupled inbetween the receiver (531) and an entropy decoder/parser (520) (“parser(520)” henceforth). In certain applications, the buffer memory (515) ispart of the video decoder (510). In others, it can be outside of thevideo decoder (510) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (510), forexample to combat network jitter, and in addition another buffer memory(515) inside the video decoder (510), for example to handle playouttiming. When the receiver (531) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (515) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (515) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such as arender device (512) (e.g., a display screen) that is not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as was shown in FIG. 5. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (520) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (520) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (520). The flow of such subgroup control information between theparser (520) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values, thatcan be input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (558). The currentpicture buffer (558) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(555), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (552) has generated to the outputsample information as provided by the scaler/inverse transform unit(551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (521) pertaining to the block, these samples can beadded by the aggregator (555) to the output of the scaler/inversetransform unit (551) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (557) from where themotion compensation prediction unit (553) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (553) in the form of symbols (521) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (557) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (556) can be a sample stream that canbe output to the render device (512) as well as stored in the referencepicture memory (557) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (520)), the current picture buffer (558) can becomea part of the reference picture memory (557), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (531) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (510) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anembodiment of the present disclosure. The video encoder (603) isincluded in an electronic device (620). The electronic device (620)includes a transmitter (640) (e.g., transmitting circuitry). The videoencoder (603) can be used in the place of the video encoder (403) in theFIG. 4 example.

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the FIG. 6example) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) is a part ofthe electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (601) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (601) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (603) may code andcompress the pictures of the source video sequence into a coded videosequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (650). In some embodiments, the controller(650) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (650) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (650) can be configured to have other suitablefunctions that pertain to the video encoder (603) optimized for acertain system design.

In some embodiments, the video encoder (603) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (630) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (633)embedded in the video encoder (603). The decoder (633) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (634). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (634) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5. Brieflyreferring also to FIG. 5, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (630) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously coded picture fromthe video sequence that were designated as “reference pictures.” In thismanner, the coding engine (632) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (640) may transmit additional datawith the encoded video. The source coder (630) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherembodiment of the disclosure. The video encoder (703) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (703) is used in theplace of the video encoder (403) in the FIG. 4 example.

In an HEVC example, the video encoder (703) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (703) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (703) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 7 example, the video encoder (703) includes the interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in FIG. 7.

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (722) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (721) is configured to determine general controldata and control other components of the video encoder (703) based onthe general control data. In an example, the general controller (721)determines the mode of the block, and provides a control signal to theswitch (726) based on the mode. For example, when the mode is the intramode, the general controller (721) controls the switch (726) to selectthe intra mode result for use by the residue calculator (723), andcontrols the entropy encoder (725) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(721) controls the switch (726) to select the inter prediction resultfor use by the residue calculator (723), and controls the entropyencoder (725) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (723) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (722) or the inter encoder (730). Theresidue encoder (724) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (724) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (703) also includes a residuedecoder (728). The residue decoder (728) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (722) and theinter encoder (730). For example, the inter encoder (730) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (722) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (725) is configured to format the bitstream toinclude the encoded block. The entropy encoder (725) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (725) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 8 shows a diagram of a video decoder (810) according to anotherembodiment of the disclosure. The video decoder (810) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (810) is used in the place of the videodecoder (410) in the FIG. 4 example.

In the FIG. 8 example, the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residue decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in FIG. 8.

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (872) or the inter decoder (880), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (880); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (872). The residual information can be subject to inversequantization and is provided to the residue decoder (873).

The inter decoder (880) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (872) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (873) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (873) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (871) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (874) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (873) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In an embodiment, the video encoders (403), (603),and (703), and the video decoders (410), (510), and (810) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques that improve contextmodeling in the block structure signaling. In some examples, thetechniques are used in audio video standard (AVS).

Generally, a video standard includes block partition strategies thatpartition a picture into blocks using various types of splits that aredefined in the video standard. For example, a block partition structurein VVC can include a quad-tree (QT) splits, binary tree (BT) splits andternary tree (TT) splits. The block partitioning strategy in VCC isreferred to as QT plus multi-type tree (MTT) (QT+MTT) strategy. Inanother example, a block partition structure in AVS can include QTsplits, BT splits and extended quad tree (EQT) splits.

A picture can be divided into an array of non-overlapping CTUs. In anexample, a CTU is a two-dimensional array of pixels or samples with upto 128×128luma samples (and corresponding chroma samples). The CTU canbe split into one or more smaller blocks using one or a combination ofthe tree splitting methods. For each of the smaller blocks resultingfrom a parent block split, a flag (referred to as a split flag) can beused to signal whether a further split using one of a plurality ofpossible tree partitions is chosen. If not, the smaller block can be aleaf node of the split and can be processed as a coding unit (CU), usingtechnologies such as prediction/transformation/quantization/entropycoding, and/or the like. Each time a split occurs, a depth of thesmaller block from a corresponding parent block can be increased by 1.The split can continue from the root of the CTU (e.g., depth=0) to acertain defined maximum depth or until a minimum allowed block size(e.g., 4 samples each side) is reached. When the defined maximum depthor the minimum allowed block size is reached, the split flag is notsignaled but can be inferred to be 0. On the other hand, at the root ofthe CTU, in some examples, the split can be inferred to be 1, such asfor an I slice, it is implicitly inferred that each 128×128 samples canbe split into four 64×64 samples at a first depth to incorporate amaximum transform size of 64×64.

The QT split can be used, for example, in HEVC, VVC and AVS. In anexample, each parent block is split in half in both horizontal andvertical directions. The resulting four smaller partitions have a sameaspect ratio as that of the parent block. In an example, a CTU isfirstly split by the QT split, for example, recursively. Each QT leafnode (e.g., in a square shape) can be further split (e.g., recursively)using BT splits, TT splits and EQT splits based on allowed split typesin the standard.

The BT split refers to a method that can divide a parent block in halfin either a horizontal direction or a vertical direction. The resultingtwo smaller partitions are half in size as compared to the parent block.

The TT split refers to a method that can divide a parent block intothree parts in either a horizontal direction or a vertical direction,for example, in a symmetrical way. A middle part of the three parts canbe twice as large as the other two parts. The resulting three smallerpartitions are ¼, ½ and ¼ in size as compared to that the parent block.

The EQT refers to a method that can divide the parent block in threeparts in either horizontal or vertical direction in a symmetrical way.The middle part of the three is twice as large as the other two parts.Further, the middle part will be split into two square parts, making thefinal result of the EQT with four partitions. After or before a flagsignaling the current coding tree will be divided using EQT split,another flag will be signaled indicating whether horizontal or verticalsplit is used.

FIG. 9 shows examples of BT splits, TT splits and EQT splits accordingto embodiments of the disclosure. A block (or a parent block) (910) canbe split using a vertical BT split into blocks (or child blocks)(911)-(912). A block (or a parent block) (920) can be split using ahorizontal BT split into blocks (or child blocks) (921)-(922). A block(or a parent block) (930) can be split using a vertical TT split intoblocks (or child blocks) (931)-(933). A block (or a parent block) (940)can be split using a horizontal TT split into blocks (or child blocks)(941)-(943). A block (or a parent block) (950) can be split using avertical EQT split into blocks (or child blocks) (951)-(954). A block(or a parent block) (960) can be split using a horizontal EQT split intoblocks (or child blocks) (961)-(964).

In an example, such as VVC, AVS and the like, a CU is a leaf node ofblock partitioning without being further split. A correspondingprediction unit (PU) and a transform unit (TU) can be of a same size asthat of the CU unless the CU size exceeds the maximum TU size (e.g.,where the residues can be split until the maximum TU size is reached).In an example, such as in HEVC, a PU size and a TU size can be smallerthan a corresponding CU size. The block partitioning operation can beconstrained by a maximum number of splits allowed (e.g., a split depth)from the CTU root and a minimum block height and width for a leaf CU. Insome examples, the smallest CU size is 4×4 in luma samples.

FIG. 10 shows a flow chat outlining a partition process (1000) used inAVS according to some embodiments. In some examples, the partitionprocess (1000) can be used for determining partition at each node in apartition structure for a CTU. The partition process (1000) starts at(S1001) for a CU at anode and proceeds to (S1005). The CU can be a CTUat a root node, can be a left node, or can be a block at an intermediatenode.

At (S1005), a determination whether to perform QT split at the CU can bemade. When the QT split is determined, the process proceeds to (S1010);otherwise, the process proceeds to (S1015).

At (S1010), QT split is performed on the CU.

At (S1015), a determination whether not to perform any split at the CUcan be made. When no split is determined, the process proceeds to(S1020); otherwise, the process proceeds to (S1025).

At (S1020), the CU is marked as a leaf node and no more further split onthe CU.

At (S1025), a determination whether to perform EQT split at the CU canbe made. When the EQT split is determined, the process proceeds to(S1030); otherwise, the process proceeds to (S1050).

At (S1030), a determination whether to perform horizontal split at theCU can be made. When the horizontal split is determined, the processproceeds to (S1035); otherwise, the process proceeds to (S1040).

At (S1035), horizontal EQT is performed on the CU.

At (S1040), vertical EQT is performed on the CU.

At (S1050), a determination whether to perform horizontal split at theCU can be made. When the horizontal split is determined, the processproceeds to (S1055); otherwise, the process proceeds to (S1060).

At (S1055), horizontal BT is performed on the CU

At (S1060), vertical BT is performed on the CU.

Besides the flexible block partitioning tools described above, forcoding of an intra slice, the coding tree structures for luma samplesand chroma samples of a CTU can be different (referred to as a dual-treestructure). Thus, chroma samples can have an independent coding treeblock structure from the collocated luma samples in the same CTU, andthus the chroma samples can have larger coding block sizes than lumasamples.

FIG. 11 shows a syntax table (1100) for block structure signalingaccording to some embodiments. In some examples, the syntax table (1100)can be used in AVS. The syntax table (1100) is a portion of recursivepartition (e.g., a function coding_tree( )) for block structure. Whenthere is no further split, a coding_unit( ) function can be called tofurther parse syntax elements inside a CU.

In the syntax table (1100), syntax elements (1101-1104) can be used forblock structure signaling at a CU node. In an example, the syntaxelement (1101) denoted by qt_split_flag can indicate a usage of quadtree split. For example, qt_split_flag equal to 1 can indicate a usageof quad tree split; and qt_split_flag equal to 0 can indicate no usageof quad tree split. A variable QtSplitFlag is set equal toqt_split_flag. In an example, when qt_split_flag is not signaled, thevariable QtSplitFlag is set equal to allowSplitQt (which is used toindicate whether QT split is allowed based on information of the CU).

Further, in an example, the syntax element (1102) denoted bybet_split_flag can indicate a usage of BT split or EQT split. Forexample, bet_split_flag equal to 1 can indicate a usage of BT split orEQT split; and bet_split_flag equal to 0 can indicate no usage of BTsplit or EQT split. A variable BetSplitFlag is set equal tobet_split_flag. When bet_split_flag is not signalled, the variableBetSplitFlag is set equal to !allowNoSplit.

In another example, the syntax element (1103) denoted bybet_split_type_flag can be used for indication of a split type selectedfrom BT and EQT. For example, bet_split_type_flag equal to 1 canindicate the EQT type; and bet_split_type flag equal to 0 can indicatethe BT type.

In another example, the syntax element (1104) denoted bybet_split_dir_flag can be used for indication of a split directionselected from horizontal direction and vertical direction. For example,bet_split_dir_flag equal to 1 can indicate the horizontal direction; andbet_dir_type flag equal to 0 can indicate the vertical direction.

According to some aspects of the disclosure, context modeling techniquescan be used for the split flags of signaling block structures. In theexample of the syntax table (1100) in FIG. 11, the syntax elements(1101-1104) are related to splitting flags of block structure. In someexamples, the context modeling of these flags is primarily based on avariety of local information, such as the depth of the current codingtree, the neighboring blocks' sizes, the neighboring blocks' splittingdepths, etc.

In some examples, a variable ctxInc is used to denote a contextassignment with values associated with context models. In an example,ctxInc for qt_split_flag can have 4 values (e.g., 0-3) respectivelyassociated with 4 context models (e.g., context models Q0-Q3), ctxIncfor bet_split_flag can have 9 values (e.g., 0-8) respectively associatedwith 9 context models (e.g., context models BE0-BE8).

In some embodiments, the assignments of the context models can bedetermined at least partially based on limitations of local information.In some examples, variables are defined based on limitations of locationinformation. In an example, a variable condL is defined based on acomparison of the current block with a left neighboring block (alsoreferred to as left block) of the current block. For example, when theleft block's height is smaller than current block's height, condL is setto 1; otherwise, condL is set to 0. In another example, a variable condAis defined based on a comparison of the current block with an aboveneighboring block (also referred to as above block) of the currentblock. For example, when the above block's height is smaller thancurrent block's height, condA is set to 1; otherwise, condA is set to 0.In another example, a variable available L is defined based onavailability of the left block. For example, when the left block isavailable (being existing and already reconstructed), availableL is setto 1; otherwise, availableL is set to 0. In another example, a variableavailableA is defined based on availability of the above block. Forexample, when the above block is available (being existing and alreadyreconstructed), availableA is set to 1; otherwise, availableA is set to0.

In an embodiment, for qt_split_flag of a current block, if the currentpicture is intra coded and current block width is 128, ctxInc is set to3 (e.g., associated with context model Q3, the context model Q3 hasrelatively high possibility of quad tree split); else if the value of(condL && availableL) && (condA && availableA) is equal to 1, ctxInc isset to 2 (e.g., associated with context model Q2); else if the value of(condL && availableL) (condA && availableA is equal to 1, ctxInc is setto 1 (e.g., associated with context model Q1); else ctxInc is set to 0(e.g., associated with context model Q0).

In another example, for bet_split_flag, if the value of (condL &&availableL) && (condA && availableA) is 1, ctxInc is set to 2 (e.g.,associated with context model BE2); else if the value of (condL &&availableL) (condA && availableA) is 1, ctxInc is set to 1 (e.g.associated with context model BE1); else, ctxInc is set to 0 (e.g.,associated with context model BE0). Then, the value of ctxInc isadjusted based on size of the current block. For example, if the numberof luma samples in current block is larger than 1024, no change is madeto ctxInc (e.g., associated with context models BE2-BE0); else if thenumber of luma samples in current block is larger than 256, the value ofctxInc is increased by 3 (e.g., associated with context model BE5-BE3);else, the value of ctxInc is increased by 6 (e.g., associated withcontext model BE8-BE6).

According to an aspect of the disclosure, in some related examples,context modeling of the block structure splitting flags of each CTU isindependent of each other. Thus, no information of previously coded CTUblock structures has been used as a predictor for the block partitioningsignaling flags of the current CTU in the related examples. The presentdisclosure provides context modeling techniques to predict blockstructure of current CTU based on previous CTUs, and the codingefficiency can be improved based on the context modeling of the relatedsplitting flags.

The proposed methods may be used separately or combined in any order.Further, each of the methods (or embodiments), encoder, and decoder maybe implemented by processing circuitry (e.g., one or more processors orone or more integrated circuits). In one example, the one or moreprocessors execute a program that is stored in a non-transitorycomputer-readable medium. In the following, the term block may beinterpreted as a prediction block, a coding block, or a coding unit,i.e. CU.

According to some aspects of the disclosure, a previously coded CTU'sblock structure is used as a reference to predict the current CTU'sblock structure. More specifically, if one coding tree block in thereference block chose to split further, it is very likely that thecoding tree block of the same location in the current CTU will choose tosplit as well. Using the information such as qt_split_flag andbet_split_flag from the reference CTU can help to establish moreaccurate context models for the syntax elements of split flags in thecurrent CTU.

In various examples, a block split structure can indicate one or more ofa maximum split depth, a minimum size of a CU, a partition or splittingmethod (e.g., QT, a MTT a BT, a TT, a EQT and the like), whether and howto partition at an intermediate node, and/or the like. Information ofthe block split structure may be pre-determined, inferred, and/orsignaled.

According to some aspects of the disclosure, a current block splitstructure for a current CTU (e.g., a CTB, a luma CTB, chroma CTB(s),and/or the like) in a current picture can be determined based onreference partitioning information of a previously decoded CTU decodedprior to the current CTU in a decoding order. The previously decoded CTUdecoded prior to the current CTU in the decoding order can be anysuitable previously decoded CTU in the current picture or in a differentpicture. For example, the reference partitioning information of thepreviously decoded CTU can include a block split structure of thepreviously decoded CTU. Alternatively, the current block split structurefor the current CTU can be determined based on reference partitioninginformation indicated by a high level header, for example, at a level(or a high level) higher than a CTU level. In an example, the referencepartitioning information in the high level header can indicate (e.g.,include) high-level block split structure(s) available for CTUsassociated with the high level (e.g., a picture level, a slice level),and an index in the high level header can indicate which of thehigh-level block split structure(s) is to be used for determining thecurrent block split structure for the current CTU. In an example, CTUsassociated with the picture level can refer to CTUs in the picture, andCTUs associated with the slice level can refer to CTUs in the slice. Thehigh-level block split structure(s) in the high level can be used by theCTUs associated with the high level, such as the CTUs in the samesequence, the CTUs in the same picture, or the like. A first subset ofCTUs associated with a same level (e.g., in the same picture, in thesame sequence) may be partitioned based on a first high-level blocksplit structure. A second subset of CTUs associated with the same level(e.g., in the same picture, in the same sequence) may be partitionedbased on a second high-level block split structure.

In some examples, the current block split structure for the currentblock can be identical to an initial block split structure (e.g., theblock split structure of the previously decoded CTU, one of thehigh-level block split structure in the high level header). Thus, nosignaling is needed in some examples since the initial block splitstructure does not need to be signaled. In some examples, a single flagis signaled to indicate a selection of the initial block splitstructure.

Alternatively, the initial block split structure can be modified toobtain the current block split structure for the current block. In someexamples, the initial block split structure can first be modified toobtain a reference block split structure. Subsequently, the referenceblock split structure can be modified to obtain the current block splitstructure. Since the initial block split structure and/or the referenceblock split structure are not signaled, fewer flags can be signaled ascompared to recursively partitioning the current CTU using methodsdescribed in FIG. 11.

In an embodiment, coding information of the current CTU can be decodedfrom a coded video bitstream. The coding information can indicatewhether the current block split structure of the current CTU is based onthe reference partitioning information. In response to the current blocksplit structure of the current CTU being based on the referencepartitioning information, the current block split structure for thecurrent CTU can be determined based on an initial block split structureindicated in the reference partitioning information. The initial blocksplit structure can be (i) the block split structure of the previouslydecoded CTU or (ii) one of the high-level block split structure(s)indicated by the high level header. Further, the current CTU can bepartitioned according to the current block split structure.

In some examples, determining (e.g., predicting) the current block splitstructure for the current CTU based on block structure information thatis not specific to the current block split structure of the current CTUcan improve coding efficiency. The block structure information canindicate block split structure(s) of previously coded CTU(s) or blocksplit structure(s) in a high level header. Fewer flag(s) can be signaledwhen the current block split structure for the current CTU is based on ablock split structure of a previously coded CTU, for example, since theblock split structure of the previously coded CTU may not needed to besignaled. Fewer flag(s) can be signaled when the current block splitstructure for the current CTU is based on a block split structure (alsoreferred to as high-level block split structure) in a high level header,for example, since the high-level block split structure can be shared byCTUs in the high level (e.g., a sequence level, a picture level). In anexample, a first number of high-level block split structures is sharedby a second number of CTUs in the same high level where the secondnumber can be much larger than the first number.

According to some aspects of the disclosure, the current block splitstructure for the current CTU can be determined (e.g., predicted) fromblock split structure(s) of other CTU(s) (e.g., previously codedCTU(s)).

The current block split structure for the current CTU can be determined(e.g., predicted) based on reference partitioning information (alsoreferred to as partitioning information) of the previously coded CTU(s).In an example, at the CTU level, a flag (e.g., a structure predictionflag (SPF) or spf_flag) is used to signal usage of prediction for thecurrent block split structure for the current CTU before parsing splitflags for the current CTU at a CTU root. If the SPF (e.g., spf_flag) istrue, a reference block split structure (or a reference CTU splitstructure, a block split structure predictor, a block partitioningstructure predictor) can be determined (e.g., derived or generated) as apredictor for the block split structure for the current CTU. If the SPF(e.g., spf_flag) is false, the current block split structure for thecurrent CTU can be coded independently, for example, based on signalingat each level to indicate if a further split is to be used and a type ofthe split. When the current block split structure for the current CTU iscoded independently, the current block split structure for the currentCTU is not dependent or is not based on a block split structure of apreviously coded CTU.

As described above, the current block split structure for the currentCTU can be based on the reference partitioning information of thepreviously coded CTU(s), for example, that are coded prior to thecurrent CTU in a coding order. In an example, at a decoder side, thepreviously coded CTU(s) are decoded prior to the current CTU in adecoding order. The reference partitioning information of the previouslycoded CTU(s) can include block split structure(s) of the previouslycoded CTU(s) used to partition the respective previously coded CTU(s).In some examples, the reference partitioning information furtherincludes a flag or an index indicating which of the block splitstructure(s) of the previously coded CTU(s) can be used for the currentblock split structure for the current CTU.

The reference partitioning information of the previously coded CTU(s)can be obtained based on spatial neighboring CTU(s) of the current CTU.Block split structure(s) of the spatial neighboring CTU(s) can be usedto predict the current block split structure of the current CTU. Thepreviously decoded CTU(s) can be the spatial neighboring CTU(s) of thecurrent CTU. The initial block split structure can be of one of thespatial neighboring CTU(s). The spatial neighboring CTU(s) can beadjacent to the current CTU.

In an example, the reference block split structure is determined basedon the spatial neighboring CTU(s), for example, the block lock splitstructure(s) of the spatial neighboring CTU(s), and subsequently, thecurrent block split structure is determined based on the reference blocksplit structure.

The current block split structure of the current CTU can be predicted byinformation (e.g., reference partitioning information) of a previouslycoded CTU. In an example, the information (e.g., the referencepartitioning information) can be from a same coding tile, coding slice,or coding tile group as that of the current CTU. Thus, a prediction ofthe current block split structure can be referred to as a spatialprediction. The spatial prediction may be determined from theinformation of spatial neighboring CTU(s) of the current CTU, forexample, a left CTU or a left coded CTU that is a CTU to the left of thecurrent CTU, a top CTU (also referred to as an above CTU) or a top codedCTU that is on top of the current CTU, and/or a top-left CTU (alsoreferred to as an above-left CTU) or a top-left coded CTU that is theCTU on a left top corner of the current CTU. Accordingly, the spatialneighboring CTU(s) of the current CTU can include but are not limitedto, the left coded CTU, the top coded CTU, and/or the top-left codedCTU.

The reference partitioning information of the previously coded CTU(s)can be obtained from a history-based buffer (or a history buffer). Thehistory-based buffer can store one or more block split structures of thepreviously coded CTU(s). In an example, the previously coded CTU(s) arein the current picture. The previously coded CTU(s) for thehistory-based buffer can include CTU(s) that are adjacent to the currentCTU and/or CTU(s) that are not adjacent to the current CTU. In someexamples, the previously coded CTU(s) for the history-based buffer caninclude CTU(s) from a picture that is different from the currentpicture. The one or more block split structures in the history-basedbuffer can be used to predict the current block split structure of thecurrent CTU. The initial block split structure of the previously decodedCTU is one of the one or more block split structures in thehistory-based buffer. In an example, the reference block split structureis determined based on one of the one or more block split structures inthe history-based buffer, and subsequently, the current block splitstructure is determined based on the reference block split structure.

In various examples, the history-based buffer for the previously codedCTU(s) in the coding order (e.g., an encoding order, a decoding order)can be maintained to store the one or more block split structures of theprevious coded CTU(s). A buffer size N (e.g., a positive integer)indicates that the history-based buffer includes N entries. Entries inthe history-based buffer can be updated. When used, an index of a blocksplit structure from the one or more block split structures in thehistory-based buffer can be signaled. Any suitable method can be usedfor index coding. In an example, the one or more block split structuresin the history-based buffer include a plurality of block splitstructures and indices for the plurality of block split structures canbe coded with suitable codewords.

In an example, a first-in-first-out (FIFO) rule is applied to maintainthe history-based buffer. Thus, the history-based buffer can keep theinformation of the block split structure(s) of N most recent codedCTU(s) in the coding order.

In an example, an entry for the most recent coded CTU can be put in alast position (or a most recent position) of the history-based buffer. Ashortest codeword can be used for index coding of the last position ofthe history-based buffer when the last position is used for predictingthe current block split structure for the current CTU.

In an example, when the entries are put into the history-based buffer,positions (e.g., locations relative to the current CTU) of thepreviously coded CTU(s) (also referred to as reference CTU(s)) arestored in the history-based buffer, for example, in addition to theblock split structure(s) of the previously coded CTU(s). When selectedfor predicting the current block split structure for the current CTU,the locations relative to the current CTU may also be considered fordesigning codewords for index coding. For example, a first index of afirst one of the plurality of block split structures for a firstpreviously coded CTU uses a shorter codeword than a second index of asecond one of the plurality of block split structures for a secondpreviously coded CTU when a first location of the first previously codedCTU is closer to the current CTU than a second location of the secondpreviously coded CTU.

The reference partitioning information of the previously coded CTU(s)can be obtained based on temporal neighboring CTU(s) of the current CTU.Block split structure(s) of the temporal neighboring CTU(s) (e.g., in areference picture that is different from the current picture) can beused to predict the current block split structure of the current CTU.The previously decoded CTU(s) can be the temporal neighboring CTU(s) ofthe current CTU, and the initial block split structure can be of one ofthe temporal neighboring CTU(s) of the current CTU.

In an example, the reference block split structure is determined basedon the temporal neighboring CTU(s), for example, the block lock splitstructure(s) of the temporal neighboring CTU(s), and subsequently, thecurrent block split structure is determined based on the reference blocksplit structure.

FIG. 12 shows an example of using temporal neighboring CTU(s) to predictthe current block split structure for a current CTU (1211) according toan embodiment of the disclosure. The current CTU (1211) is in a currentpicture (1201). In an example, a reference picture (1202) is determined.For example, the reference picture (1202) is a collocated picture usedto derive a TMVP MV predictor. A CTU (1221) is a collocated CTU of thecurrent CTU (1211). The temporal neighboring CTU(s) used to predict thecurrent block split structure for the current CTU (1211) can include anysuitable CTU(s) in the reference picture (1202). In an example, thetemporal neighboring CTU(s) include the collocated CTU (1221) and one ormore of neighboring CTUs (1222)-(1229) of the collocated CTU (1221). Thetemporal neighboring CTU(s) can also be referred to as reference CTU(s).

In an example, the collocated CTU (1221) is assigned the shortestcodeword for index coding, and the eight neighboring CTUs (1222)-(1229)of the collocated CTU (1221) are assigned with fixed length (e.g.,3-bit) codewords for index coding. In an example, the codewords for theCTUs (1221)-(1229) are 1, 000, 001, 010, 011, 100, 101, 110, and 111,respectively, as shown in FIG. 12. Accordingly, when the block splitstructure of the CTU (1221) is used to predict the current block splitstructure of the current CTU (1211), an index of ‘1’ can be signaled.Alternatively, when the block split structure of the CTU (1228) is usedto predict the current block split structure of the current CTU (1211),an index of ‘110’ can be signaled.

As described above, the reference partitioning information can beindicated by the high level header. Information (e.g., the referencepartitioning information) of the block split structure(s) (also referredto as high-level block split structure(s)) can be stored in the highlevel header, such as in a slice header, a PPS, a SPS, or the like. Thereference partitioning information can include the high-level blocksplit structure(s). In an example, the high level is higher than the CTUlevel. When the reference partitioning information including thehigh-level block split structure(s) is used at the CTU level, an indexindicating one of the high-level block split structure(s) can be sent.In an example, the index indicates a position of the one of thehigh-level block split structure(s) in a storage buffer (e.g., the highlevel header).

In an example, the high-level block split structure(s) include aplurality of block split structures in the high level header, and theinitial block split structure is one of the plurality of block splitstructures and is indicated by an index included in the high levelheader.

The high-level block split structure(s) can be used to predict thecurrent block split structure of the current CTU. In an example, thereference block split structure is determined based on the high-levelblock split structure(s), and subsequently, the current block splitstructure is determined based on the reference block split structure.

For each coded slice or coded picture, the reference partitioninginformation (also referred to as predictor information) in the storagebuffer may be updated (e.g., partially or completely) for the codedslice or the coded picture. For example, predictors (e.g., high-levelblock split structures) A1-A10 can be derived from a PPS of a firstpicture. The storage buffer can include information of the predictorsA1-A10 the first picture. CTUs in the first picture can refer to thepredictors A1-A10 to obtain block split structures for the respectiveCTUs in the first picture. When parsing a PPS of a second picture, forexample, after parsing the PPS of the first picture, an indication ofkeeping the predictors A6-A10 is received, together with information ofpredictors B1-B5. The storage buffer can include the information of thepredictor B1-B5 and the information of the predictors A6-A10 for thesecond picture. CTUs in the second picture can refer to the predictorsA6-A10 and the predictor B1-B5 to obtain block split structures for therespective CTUs in the second picture.

As described above, the reference block split structure for the currentCTU can be determined based on the reference partitioning information,such as the block split structure(s) of the previously coded CTU(s), thehigh-level block split structure(s) indicated in the high level header,or the like.

In an example, an initial block split structure (e.g., the block splitstructure of the previously coded CTU, the high-level block splitstructure indicated (e.g., included) in the high level header, or thelike) is used directly as the reference block split structure, and thusthe reference block split structure is the initial block splitstructure. Alternatively, the initial block split structure (e.g., theblock split structure of the previously coded CTU, the high-level blocksplit structure indicated (e.g., included) in the high level header, orthe like) can be processed or modified to obtain the reference blocksplit structure, and thus the reference block split structure isdifferent from the initial block split structure. Certain partitioninginformation in the initial block split structure can be removed. Certainpartitioning information in the initial block split structure can beapproximated or replaced by new partitioning information.

FIG. 13 shows an example where a block split structure for a previouslycoded CTU (1301) is modified to obtain a reference block split structurefor a CTU (1302) according to an embodiment of the disclosure. Apreviously coded CTU (1301) is split with a QT split into four 64×64blocks (1311)-(1314) at a first level (e.g., a CTU level). A top-right64×64 block (1312) is further split into smaller partitions while thethree 64×64 blocks (1311), (1313), and (1314) are not split, resultingin the block split structure for the previously coded CTU (1301). Whenthe block split structure for the previously coded CTU (1301) is used asa predictor, the detailed split structure of the top-right 64×64 block(1312) can be removed and then represented by a single QT split (1330)at a top-right 64×64 block (1322), as shown in the reference block splitstructure for the CTU (1302). The reference block split structure forthe CTU (1302) can split a CTU (1302) at a CTU level with a QT splitinto four 64×64 blocks (1321)-(1324). Subsequently, the top-right 64×64block (1322) is further split into four smaller blocks (1331)-(1334) bythe QT split (1330). In an example, the reference block split structure(e.g., the reference block split structure for the CTU (1302)) modifiedfrom the initial block split structure (e.g., the block split structurefor the previously coded CTU (1301)) is simplified, and thus can have aless split depth and/or a less number of leaf nodes than those of theinitial block split structure. Modifying the initial block splitstructure to obtain the reference block split structure can beadvantageous. When the reference block split structure is simplified,such as shown in FIG. 13, there is less information to store for thereference block split structure, and thus saving memory space. Further,various block split structures for CTUs may be represented by a smallernumber of variations as the reference block split structures.

Any suitable modification rules can be applied to obtain the referenceblock split structure. In an example, only QT splits can be used orallowed in the reference block split structure, such as shown in thereference block split structure for the CTU (1302). In an example, whenonly the QT splits are allowed in the reference block split structure,only one bit or one flag is used to indicate the QT split, and no otherbit is needed to indicate how to implement the QT split, thus improvingthe coding efficiency.

In an example, QT split(s) and BT split(s) can be used in the referenceblock split structure, and other splitting method(s) are disallowed inthe reference block split structure.

In an embodiment, only QT splits can be used in the reference blocksplit structure. Further, up to L split level(s) is allowed in thereference block split structure. L can be an integer, such as 0, 1, 2,or the like. In an example, L is 0, and a CTU predicted with thereference block split structure includes a single block, for example, of128×128 without being split. In an example, L is 1, the CTU predictedwith the reference block split structure can be a single block of128×128 samples without being split or include four 64×64 blocks withone split. In an example, L is 2, the CTU predicted with the referenceblock split structure can be a single block of 128×128 samples orinclude four 64×64 blocks where each 64×64 block may be further splitinto four 32×32 blocks.

In an example, the split level L is determined by a complexity of ablock or a region, such as a 64×64 region. Referring to FIG. 13, thefour 64×64 regions or blocks (1321)-(1324) in the reference block splitstructure can have different split depths for the QT split. For example,the top-right 64×64 region (1322) can be further split one time whilethe other three 64×64 regions are not split and are maintained at a64×64 level. Accordingly, the split depth L for the region (1322) is 2,and the split depth L for the regions (1321), (1323), and (1324) are 1.

In an embodiment, up to 2 levels of QT splitting from the CTU root areallowed for representing the reference CTU's block structure. In someexamples, a variable refCtuSplitStructure is used to represent the QTsplit structure.

FIG. 14 shows an example of using the variable refCtuSplitStructure forrepresenting the QT split structure. In the FIG. 14 example, two levelsof QT splitting from the CTU root are allowed. The variablerefCtuSplitStructure includes 5 binary bits. The first binary bit isused to represent whether QT split is used at the first level of the QTsplit structure of the root CTU, and the four other binary bits are usedto represent the second level of QT split structure of the four subblocks of the root CTU when the first level QT split is used. Forexample, when no QT split has been used on the root CTU, five binarybits “00000” can represent the QT structure. When QT split is used onthe root CTU, the first bit of the five binary bits is set to “1”. TheCTU is split into four sub blocks that can be referred to as top leftsub block, top right sub block, bottom left sub block and bottom rightsub block. Then, the second bit of the five bits is used to representwhether QT split is used on the top left sub block; the third bit of thefive bits is used to represent whether QT split is used on the top rightsub block; the fourth bit of the five bits is used to represent whetherQT split is used on the bottom left sub block; the fifth bit of the fivebits is used to represent whether QT split is used on the bottom rightsub block.

According to some aspects of the disclosure, the reference block splitstructure can be determined based on the initial block split structure,for example, indicated in the reference partitioning information. Thecurrent block split structure can be determined based on the referenceblock split structure.

If predicted by the reference block split structure, the current blocksplit structure for the current CTU can use the reference block splitstructure directly. Alternatively, the reference block split structurecan be further modified to obtain the current block split structure(also referred to as a final block split structure) for the current CTU.Whether to use the reference block split structure with or withoutmodification can be signaled or pre-determined.

According to aspects of the disclosure, the coding information caninclude a flag indicating whether the reference block split structure ismodified to obtain the current block split structure of the current CTU.Thus, whether the reference block split structure is modified to obtainthe current block split structure of the current CTB can be determinedbased on the flag

In an example, the flag is used to signal whether the reference blocksplit structure is used with or without modification. If the referenceblock split structure is used with modification, further modification(s)can be signaled after the prediction with the reference block splitstructure.

In an example, at each predicted child node, a split flag is used tosignal whether a further split is to be used on top of the currentprediction. If the further split is to be used, a type of the split canbe signaled.

In an example, the current CTU is first partitioned according to thereference block split structure to obtain a plurality of child nodes.Subsequently, the recursive partitioning can be applied to each of thechild node to further divide the child node if necessary. For example,at each child node, a split flag is used to signal whether a furthersplit is to be used. If the further split is to be used, a type of thesplit can be signaled. Accordingly, the current CTU is partitioned usinga combination of the reference block split structure and signaling, andthus reducing signaling overhead since the reference block splitstructure is not signaled.

In an example, the current block split structure for the current CTU canuse the reference block split structure directly (e.g., withoutmodification). Accordingly, the current block split structure for thecurrent CTU can be identical to the reference block split structure. Nomore split flags are signaled or inferred, and thus the codingefficiency can be improved. For example, the reference block splitstructure is four 64×64 blocks. If the reference block split structureis used, the current CTU can be split into four 64×64 blocks and thereis no need to check if any of the 64×64 blocks is to be split further.

According to some aspects of the disclosure, the reference block splitstructure can be modified to obtain the current block split structurefor the current CTU. In an example, the reference block split structureis used to determine the current block split structure for the currentCTU, the current CTU can first be partitioned based on the referenceblock split structure and potentially add more split flags on top of thepredicted structure. For example, the reference block split structure isfour 64×64 blocks. If the reference block split structure is used, thecurrent CTU can be split into four 64×64 blocks (also referred to aschild nodes) first. For each of the child nodes (e.g., each 64×64block), a split flag may be signaled or derived (e.g., inferred) toindicate whether the child node is to be split further or not. If thechild node is to be split further, additional information (e.g.,including a type of the split and a split direction (e.g., a verticaldirection, a horizontal direction) can be signaled. Each of theresulting child nodes can be processed recursively. Though theadditional information may be signaled, modifying the reference blocksplit structure to obtain the current block split structure may improvecoding efficiency since the reference block split structure may not needto be signaled.

According to some aspects of the disclosure, context modeling techniquesare used for the split flags of the current CTU, and the reference CTU'ssplit structure and local information of the current CTU can be used todetermine the context models for the split flags of current CTU.

Specifically, in AVS, the context modelling of the split flags for thecurrent CTU, such as qt_split_flag, ebt_split_flag, and the like cantake consideration of the information of the reference CTU's blockstructure (also referred to as split structure). While qt_split_flag andebt_split_flag are used as examples for illustrating the contextmodelling techniques, the illustrated context modelling techniques canbe suitably used on other split flags, such as bet_split_type_flag,bet_split_dir_flag and the like.

In the following description, a term “collocated position” refers to aposition in the reference CTU, the position in the reference CTUrelative to the top left corner of the reference CTU is the same as thetop-left corner of a current CU relative to the top-left corner of thecurrent CTU. When selecting a context model for a split flag (such ascu_split_flag, ebt_split_flag) signalling of a current CU in the currentCTU, the split structure of collocated position in the reference CTU isused to determine the context model.

In some embodiments, the feature of the split flag prediction from thereference CTU can be enabled or disabled at a high level syntax, such asinformation in SPS, PPS, picture header, slice header, tile/tile groupheader or sub-picture header, etc.

In some examples, a variable denoted by “allowSplitRef” is used toindicate the split information from the reference CTU, and can be usedas split prediction information. For example, when the split predictionfrom the reference CTU indicates that the current block (coding tree)'ssplit flag(s) is likely to be TRUE, then the variable “allowSplitRef”for the current block (coding tree) is set to be 1; otherwise, thevariable “allowSplitRef” for the current block (coding tree) is set tobe 0.

The present disclosure provides some techniques for the indication ofsplit prediction from the reference CTU. In an embodiment, for the firstlevel split from the current CTU root, the reference CTU's informationof whether the reference CTU has split at its first level can bechecked. If the reference CTU has (QT) split at CTU root, then theallowSplitRef is set to 1 for the current CTU at first level. Otherwise,allowSplitRef is set to 0.

In another embodiment, for the second level split(s) from the CTU root,depending on where the current coding tree is located, the splitinformation at the similar position of the reference CTU can be checked.In an example, when the top-left corner of he current block relative tothe CTU root's top-left corner is (x0, y0), then the block (codingtree)'s split information at the same offset (x0, y0) (referred to asreference location) from the reference CTU will be checked. When thereference location has second level split, then the allowSplitRef is setto 1 for the current CTU at second level at location (x0, y0).Otherwise, allowSplitRef is set to 0 at location (x0, y0).

In some embodiments, the offset of current block's top-left locationfrom its CTU root (top left corner of the current CTU) is (x0, y0)=(x %ctuSizeY, y % ctuSizeY), where (x, y) is the current block's top-leftlocation in the picture, ctuSize Y is the width/height of the CTU. Insome examples, ctuSizeY=128, then (x0/64, y0/64) can be used to refer toone of the 4 64×64 regions in the CTU.

In some embodiments, split information of the reference CTU can besuitably stored. In an embodiment, an arrayrefSplitStructure[level-1][location] is used to store split informationfrom the reference CTU. The array has two dimensions, the firstdimension is the indication of split level and the second dimension isthe indication of the relative location inside the CTU. For example, forthe first level split (from the CTU root), allowSplitRef is set equal torefSplitStructure[0][0]; for the second level split (from one of 64×64regions), allowSplitRef for that region is set equal torefSplitStructure[1][x0/64+2xy0/64].

In some examples, when a CTU is encoded or decoded, the splitinformation of the CTU can be stored in a similar fashion as the aboverefSplitStructure array for future reference. In some examples, thereference CTU's split structure is stored in other form, and can bederived into the similar fashion as in the above refSplitStructurearray.

According to some embodiments, the variable allowSplitRef is then usedin the context model selection for the split flags of the current CTU.

In some embodiments, the existing contexts for the split flags aredoubled, using additional 1 dimension of the split prediction from thereference CTU, denoted as allowSplitRef. In some examples, the existingcontext models form a first set of context models, and a second set ofcontext models associated with the variable allowSplitRef being true canbe added.

In an example, the variable ctxInc is used and assigned with a valueassociated with the selected context model. The assignment of ctxInc isbased on location information of the current CTU and the reference splitinformation of the reference CTU.

Some embodiments are described in detail. In the following description,“A” denotes an above block of the current block, “L” denotes a leftblock of the current block. Further, a variable condL is defined basedon a comparison of the current block with a left neighboring block (alsoreferred to as left block) of the current block. For example, when theleft block's height is smaller than current block's height, condL is setto 1; otherwise, condL is set to 0. In another example, a variable condAis defined based on a comparison of the current block with an aboveneighboring block (also referred to as above block) of the currentblock. For example, when the above block's height is smaller thancurrent block's height, condA is set to 1; otherwise, condA is set to 0.In another example, a variable available L is defined based onavailability of the left block. For example, when the left block isavailable (being existing and already reconstructed), availableL is setto 1; otherwise, availableL is set to 0. In another example, a variableavailableA is defined based on availability of the above block. Forexample, when the above block is available (being existing and alreadyreconstructed), availableA is set to 1; otherwise, availableA is set to0.

In some embodiments, two sets of context models are formed. In anexample, the first set of context models is associated with no split atthe collocated positon of the reference block (e.g., allowSplitRef being0) and the second set of context models is associated with splitting atthe collocated position of the reference block (e.g., allowSplitRefbeing 1).

In an embodiment, for qt_split_flag, the ctxInc is determined based ontwo steps. In a first step, ctxInc is set according to local informationas described in some above embodiments. For example, ctxInc is set to 3if the current picture is intra coded and current block width is 128;else if (condL && availableL) && (condA && availableA) is true, ctxIncis set to 2; else if (condL && availableL)∥(condA && availableA) istrue, ctxInc is set to 1; otherwise, ctxInc is set to 0. In a secondstep, modifications are made to txInc based on the value ofallowSplitRef. In an example, ctxInc is increased by 4×allowSplitRef.Thus, when allowSplitRef is “0”, no change is made to the selection ofcontext models; and when allowSplitRef is “1”, the selection of contextmodels is from a second set of context models.

In another embodiment, for bet_split_flag, ctxInc is determined in threesteps. In the first step, ctxInc is set according to local informationas described in some above embodiments. For example, if (condL &&availableL) && (condA && availableA) is true, ctxInc is set to 2; elseif (condL && availableL) (condA && availableA) is true, ctxInc is set to1; otherwise, ctxInc is set to 0.

In the second step, ctxInc is modified based on the size of the currentblock. For example, when the number of luma samples in current block islarger than 1024, no change to ctxInc; else if the number of lumasamples in current block is larger than 256, ctxInc is increased by 3;else, ctxInc is increased by 6.

In the third setp, ctxInc is further modified based on the splitinformation of the reference CTU. In an example, ctxInc is increased by9×allowSplitRef.

In some embodiments, the split prediction from the reference CTU,denoted as allowSplitRef, is used to add some additional conditions tocontexts selection.

In an embodiment, when allowSplitRef is true, there is a highpossibility that QT split will be used. Thus, allowSplitRef being truecan be added as a condition to select a context model with a highpossibility for QT splitting. In an example, for qt_split_flag, contextmodel Q3 has a higher possibility for QT splitting. When the currentpicture is intra coded and current block width is 128, or allowSplitRefis equal to true (value 1), ctxInc is set to 3; else if (condL &&availableL) && (condA && availableA) is true, ctxInc is set to 2; elseif (condL && availableL) (condA && availableA) is true, ctxInc is set to1; else, ctxInc is set to 0.

In another embodiment, for bet_split_flag, the second step is suitablymodified. For example, in the first step, if (condL && availableL) &&(condA && availableA) is true, ctxInc is set to 2; else if (condL &&availableL) (condA && availableA) is true, ctxInc is set to 1; else,ctxInc is set to 0. In the second step, if the number of luma samples incurrent block is larger than 1024, or allowSplitRef is true (value 1),no change to ctxInc; else if the number of luma samples in current blockis larger than 256, ctxInc is increased by 3; else, ctxInc is increasedby 6.

In some embodiments, the split flag prediction, or variableallowSplitRef may be used for a certain levels (depths) of the splits.For other levels (depths), the split flag prediction will not be used,or variable allowSplitRef is set to be 0 for the other levels. In someexamples, the split flag prediction may only be used in the first twolevels of split flags from the CTU roots.

FIG. 15 shows a flow chart outlining a process (1500) according to anembodiment of the disclosure. The process (1500) can be used topartition a current CTU (e.g., a current CTB, a current luma CTB,current chroma CTB(s)), so as to generate a prediction block for acurrent block under reconstruction in the current CTU. In variousembodiments, the process (1500) are executed by processing circuitry,such as the processing circuitry in the terminal devices (310), (320),(330) and (340), the processing circuitry that performs functions of thevideo encoder (403), the processing circuitry that performs functions ofthe video decoder (410), the processing circuitry that performsfunctions of the video decoder (510), the processing circuitry thatperforms functions of the video encoder (603), and the like. In someembodiments, the process (1500) is implemented in software instructions,thus when the processing circuitry executes the software instructions,the processing circuitry performs the process (1500). The process (1500)starts at (51501) and proceeds to (S1510).

At (S1510), coding information of a current CTU in a current picture canbe decoded from a coded video bitstream. In some embodiments, the codinginformation indicates that partition information of a reference CTU canbe used to predict the split structure of the current CTU.

At (S1520), a context model for a split flag associated with a currentblock within the current CTU is determined at least partially based onsplit information of a corresponding block in the reference CTU for thecurrent CTU. The split flag associated with the current block isindicative of split information of the current block.

In the example of AVS, split flag can be one of a quad tree (QT) splitflag indicating whether a QT split is used; a binary or extended quadtree (BET) split flag indicating whether a BET split is used; a BETsplit type flag indicating a split type selected from a binary tree (BT)split and an extended quad tree (EQT) split; and a BET split directionflag indicting a direction selected from a vertical direction and ahorizontal direction.

It is noted that the reference CTU can be any suitable reference CTUthat has been decoded. In an example, the reference CTU is a spatialneighbor of the current CTU. In another example, the reference CTU has asplit structure stored in a history buffer. In another example, thereference CTU is in a different picture from the current CTU. In anotherexample, the reference CTU is indicated in a high level header.

In some embodiments, a relative position of a top left corner of thecurrent block with reference to a top left corner of the current CTU isthe same as a relative position of a top left corner of thecorresponding block with reference to a top left corner of the referenceCTU.

In some embodiments, the context model is selected from a first set ofcontext models in response to the split information of the correspondingblock indicating further splitting of the corresponding block; and thecontext model is selected from a second set of context models inresponse to the split information of the corresponding block indicatingno further splitting of the corresponding block.

In some embodiments, when the split information of the correspondingblock indicates further splitting of the corresponding block, thecontext model is selected with a largest possibility for furthersplitting the current block.

In some embodiments, the usage of the split information of thecorresponding block in the determination of the context model can bedisabled in response to a partition depth being deeper than a threshold.For example, when the partition depth from the root is deeper than two,the usage of the split information of the corresponding block in thedetermination of the context model can be disabled.

In some embodiments, the split information of the corresponding block isdetermined based on a two levels quad tree split structure with a firstbit indicting QT splitting information of the reference CTU and fourother bits respectively indicating splitting information of four subblocks of the reference CTU.

In an embodiment, the context model for the split flag is determinedbased on a slice type of the current CTU. In another embodiment, thecontext model for the split flag is determined based on the referenceCTU in response to a split depth of the current CTU is below athreshold.

At (S1530), the split flag is determined based on the context model.

At (S1540), the current block is decoded based on the split flag that isdetermined based on the context model. Then the process proceeds to(S1599) and terminates.

Embodiments in the disclosure may be used separately or combined in anyorder. Further, each of the methods (or embodiments), an encoder, and adecoder may be implemented by processing circuitry (e.g., one or moreprocessors or one or more integrated circuits). In one example, the oneor more processors execute a program that is stored in a non-transitorycomputer-readable medium.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 16 shows a computersystem (1600) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 16 for computer system (1600) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1600).

Computer system (1600) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1601), mouse (1602), trackpad (1603), touchscreen (1610), data-glove (not shown), joystick (1605), microphone(1606), scanner (1607), camera (1608).

Computer system (1600) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1610), data-glove (not shown), or joystick (1605), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1609), headphones(not depicted)), visual output devices (such as screens (1610) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability-some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1600) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1620) with CD/DVD or the like media (1621), thumb-drive (1622),removable hard drive or solid state drive (1623), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1600) can also include an interface (1654) to one ormore communication networks (1655). Networks can for example bewireless, wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G,LTE and the like, TV wireline or wireless wide area digital networks toinclude cable TV, satellite TV, and terrestrial broadcast TV, vehicularand industrial to include CANBus, and so forth. Certain networkscommonly require external network interface adapters that attached tocertain general purpose data ports or peripheral buses (1649) (such as,for example USB ports of the computer system (1600)); others arecommonly integrated into the core of the computer system (1600) byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). Using any of these networks, computersystem (1600) can communicate with other entities. Such communicationcan be uni-directional, receive only (for example, broadcast TV),uni-directional send-only (for example CANbus to certain CANbusdevices), or bi-directional, for example to other computer systems usinglocal or wide area digital networks. Certain protocols and protocolstacks can be used on each of those networks and network interfaces asdescribed above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1640) of thecomputer system (1600).

The core (1640) can include one or more Central Processing Units (CPU)(1641), Graphics Processing Units (GPU) (1642), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1643), hardware accelerators for certain tasks (1644), graphics adapter(1650), and so forth. These devices, along with Read-only memory (ROM)(1645), Random-access memory (1646), internal mass storage such asinternal non-user accessible hard drives, SSDs, and the like (1647), maybe connected through a system bus (1648). In some computer systems, thesystem bus (1648) can be accessible in the form of one or more physicalplugs to enable extensions by additional CPUs, GPU, and the like. Theperipheral devices can be attached either directly to the core's systembus (1648), or through a peripheral bus (1649). In an example, a display(1610) can be connected to the graphics adapter (1650). Architecturesfor a peripheral bus include PCI, USB, and the like.

CPUs (1641), GPUs (1642), FPGAs (1643), and accelerators (1644) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1645) or RAM (1646). Transitional data can be also be stored in RAM(1646), whereas permanent data can be stored for example, in theinternal mass storage (1647). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1641), GPU (1642), massstorage (1647), ROM (1645), RAM (1646), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1600), and specifically the core (1640) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1640) that are of non-transitorynature, such as core-internal mass storage (1647) or ROM (1645). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1640). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1640) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1646) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1644)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

-   JEM: joint exploration model-   VVC: versatile video coding-   BMS: benchmark set-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component Interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit-   QT: Quaternary-Tree-   BT: Binary-Tree-   TT: Ternary-Tree-   MTT: Multi-Type Tree-   SPS: Sequence Parameter Set-   PPS: Picture Parameter Set

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding in a decoder,comprising: receiving, by a processor, coded information of a currentcoding tree unit (CTU) from a coded video bitstream; determining, by theprocessor, a context model for a split flag associated with a currentblock within the current CTU at least partially based on splitinformation of a corresponding block in a reference CTU for the currentCTU, the split flag associated with the current block being indicativeof split information of the current block; determining, by theprocessor, the split flag based on the context model; and decoding, bythe processor, the current block based on the split flag that isdetermined based on the context model.
 2. The method of claim 1, whereinthe split flag comprises at least one of: a quad tree (QT) split flagindicating whether a QT split is used; a binary or extended quad tree(BET) split flag indicating whether a BET split is used; a BET splittype flag indicating a split type selected from a binary tree (BT) splitand an extended quad tree (EQT) split; and a BET split direction flagindicting a direction selected from a vertical direction and ahorizontal direction.
 3. The method of claim 1, further comprising atleast one of: determining, by the processor, a first reference CTU thatis a spatial neighbor of the current CTU as the reference CTU;determining, by the processor, a second reference CTU with a splitstructure stored in a history buffer as the reference CTU; determining,by the processor, a third reference CTU in a different picture from thecurrent CTU as the reference CTU; and determining, by the processor andbased on information in a high level header, a fourth reference CTU asthe reference CTU.
 4. The method of claim 1, wherein a relative positionof a top left corner of the current block with reference to a top leftcorner of the current CTU is the same as a relative position of a topleft corner of the corresponding block with reference to a top leftcorner of the reference CTU.
 5. The method of claim 1, furthercomprising: selecting, by the processor, the context model from a firstset of context models in response to the split information of thecorresponding block indicating further splitting of the correspondingblock; and selecting, by the processor, the context model from a secondset of context models in response to the split information of thecorresponding block indicating no further splitting of the correspondingblock.
 6. The method of claim 1, further comprising: selecting, by theprocessor, the context model with a largest possibility for furthersplitting the current block in response to the split information of thecorresponding block indicating further splitting of the correspondingblock.
 7. The method of claim 1, further comprising: disabling, by theprocessor, a usage of the split information of the corresponding blockin the determination of the context model in response to a partitiondepth being deeper than a threshold.
 8. The method of claim 1, furthercomprising: determining, by the processor, the split information of thecorresponding block based on a two levels quad tree split structure witha first bit indicting QT splitting information of the reference CTU andfour other bits respectively indicating splitting information of foursub blocks of the reference CTU.
 9. The method of claim 1, furthercomprising: determining, by the processor, the context model for thesplit flag based on a slice type of the current CTU.
 10. The method ofclaim 1, further comprising: determining, by the processor, the contextmodel for the split flag based on the reference CTU in response to asplit depth of the current CTU is below a threshold.
 11. An apparatusfor video decoding, comprising: processing circuitry configured to:receive coded information of a current coding tree unit (CTU) from acoded video bitstream; determine a context model for a split flagassociated with a current block within the current CTU at leastpartially based on split information of a corresponding block in areference CTU for the current CTU, the split flag associated with thecurrent block being indicative of split information of the currentblock; determine the split flag based on the context model; and decodethe current block based on the split flag that is determined based onthe context model.
 12. The apparatus of claim 11, wherein the split flagcomprises at least one of: a quad tree (QT) split flag indicatingwhether a QT split is used; a binary or extended quad tree (BET) splitflag indicating whether a BET split is used; a BET split type flagindicating a split type selected from a binary tree (BT) split and anextended quad tree (EQT) split; and a BET split direction flag indictinga direction selected from a vertical direction and a horizontaldirection.
 13. The apparatus of claim 11, wherein the processingcircuitry is configured to determine at least one of: a first referenceCTU that is a spatial neighbor of the current CTU as the reference CTU;a second reference CTU with a split structure stored in a history bufferas the reference CTU; a third reference CTU in a different picture fromthe current CTU as the reference CTU; and a fourth reference CTU as thereference CTU based on information in a high level header.
 14. Theapparatus of claim 11, wherein a relative position of a top left cornerof the current block with reference to a top left corner of the currentCTU is the same as a relative position of a top left corner of thecorresponding block with reference to a top left corner of the referenceCTU.
 15. The apparatus of claim 11, wherein the processing circuitry isconfigured to: select the context model from a first set of contextmodels in response to the split information of the corresponding blockindicating further splitting of the corresponding block; and select thecontext model from a second set of context models in response to thesplit information of the corresponding block indicating no furthersplitting of the corresponding block.
 16. The apparatus of claim 11,wherein the processing circuitry is configured to: select the contextmodel with a largest possibility for further splitting the current blockin response to the split information of the corresponding blockindicating further splitting of the corresponding block.
 17. Theapparatus of claim 11, wherein the processing circuitry is configuredto: disable a usage of the split information of the corresponding blockin the determination of the context model in response to a partitiondepth being deeper than a threshold.
 18. The apparatus of claim 11,wherein the processing circuitry is configured to: determine the splitinformation of the corresponding block based on a two levels quad treesplit structure with a first bit indicting QT splitting information ofthe reference CTU and four other bits respectively indicating splittinginformation of four sub blocks of the reference CTU.
 19. The apparatusof claim 11, wherein the processing circuitry is configured to:determine the context model for the split flag based on a slice type ofthe current CTU.
 20. The apparatus of claim 11, wherein the processingcircuitry is configured to: determine the context model for the splitflag based on the reference CTU in response to a split depth of thecurrent CTU is below a threshold.